This invention is generally directed to imaging members, and more specifically the present invention relates to the selection of certain novel polyurethanes which function as protective overcoatings for photoresponsive imaging members, especially inorganic imaging members. In one embodiment, the present invention relates to an imaging member comprised of an inorganic photoconductive composition, and coated thereover certain polyurethane polymers. Also, in another embodiment of the present invention, there are provided imaging members comprised of a photogenerating layer, a hole transport layer, and thereover as a protective overcoating specific polyurethane polymers. The aforementioned imaging members are useful in electrophotographic printing and imaging processes, and in particular, can be selected for the generation of latent images in electrostatic imaging systems.
The polyurethanes of the present invention, when selected for the imaging members disclosed herein, perform a variety of functions inclusive of providing protection for the aforementioned members from abrasive, physical, and chemical contamination. Accordingly, thus for example, the specific overcoating polyurethanes of the present invention permit the resulting imaging member to be resistant to ozone and other chemical substances produced by corona charging devices. Also, the polyurethane overcoatings of the present invention substantially eliminate undesirable scratching of the imaging members involved, and further these coatings can function as release materials permitting the excellent removal and transfer of toner images. Furthermore, the polyurethane coatings of the present invention can be easily formulated as discrete layers and remain essentially nonreactive to the ink/solvent formulations utilized for certain liquid ink xerographic development processes. Moreover, the protective overcoatings of the present invention are nontoxic and are, therefore, inert to users of the device. Additionally, the protective polyurethane overcoatings are not sensitive to changes in many environmental conditions (humidity and temperature), thus ensuring the electrical performance of the protected imaging members for numerous imaging cycles.
It is known that the application of protective coatings to certain photoconductive materials, particularly inorganic photoconductive materials, is designed primarily for the purpose of extending the useful life of the resulting devices. Generally, for these coatings to provide the desired protection they should possess certain mechanical properties, and are usually applied in a substantially uniform thickness. Additionally, the coating material should be selected so as to not adversely effect the photoelectric properties of the photoreceptor, for example, the coating should not appreciably inject charges in the dark. The protective coatings should also not conduct laterally on the overcoated surface thereof. Further, in some applications the coating must be transparent, and possess a dark resistivity at least equal to the dark resistivity of the photoconductive material. For example, photoconductive materials such as selenium have a resistivity in the dark of 10.sup.10 to 10.sup.12 ohm-cm, thus the dark resistivity of the protective coating should usually be in this range when it is used as a protectant for selenium. In addition, the coatings should not be sensitive to changes in humidity and certain temperature ranges otherwise the photoelectric properties of the protected photoreceptors can be altered.
With regard to vitreous selenium, one of the most widely used photoconductive materials, it suffers from two serious defects, namely, its spectral response is somewhat toward the blue or near ultraviolet, and the preparation of uniform films of vitreous selenium has required highly complex processes wherein critical parameters are involved. Accordingly, from a commercial economic aspect, it is important that xerographic selenium devices be utilized for numerous imaging cycles. The overcoatings of the present invention enable this and other objectives to be achieved.
Deterioration by mechanical abrasion attendant to the developing and the cleaning processes, wherein in one cleaning process a rapidly rotating brush contacts the photoconductive surface for the purpose of removing therefrom any residual developer particles adhering thereto subsequent to the transfer step, has been observed in selenium. In addition to mechanical abrasion, the selenium photoreceptor may be subjected to intense heat, which over a period of time adversely effects its photoconductivity. Accordingly, and for other reasons inclusive of preventing crystallization of selenium upon exposure to chemical vapors, various protective coatings, or overcoatings have been applied to selenium devices. Thus, there is described in U.S. Pat. No. 3,397,982 an electrostatic member comprised of a photoconductive layer including an inorganic glass material, and thereover an overcoating comprised of various oxides, such as germanium oxides, the oxides of vanadium, and silicon dioxide.
Additionally, in U.S. Pat. No. 2,886,434 there are disclosed processes for the protection of selenium photoconductive substances with a thin transparent film of a material having electrical characteristics equal to selenium. Examples of materials disclosed in the U.S. Pat. No. 2,886,434 as a protective layer for selenium include zinc sulfide, silca, various silicates, alkaline earth fluorides, and the like. Furthermore, there is disclosed in U.S. Pat. No. 2,879,360 a photoconductor comprising a support substrate, a layer of photoconductive material, and as a protectant a thin film of silicon dioxide superimposed upon the photoconductive layer.
Also, there are illustrated in the prior art photoresponsive devices comprised of a conductive substrate overcoated with a hole injecting layer, which in turn is overcoated with a hole transport layer, followed by a carrier generating layer, and an insulating organic resin as a top coating. These devices have been found to be very useful in various imaging systems, and have the advantage that high quality images are obtained with the overcoating acting primarily as a protectant. Another similar overcoated photoresponsive device is comprised of a conductive substrate layer, a generating layer, and a transport layer. In such devices, the generating layer can be overcoated on the transport layer, or the transport layer may be overcoated on the generating layer. Examples of such devices are described in U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference.
Additionally, there are illustrated in U.S. Pat. No. 4,423,131, the disclosure of which is totally incorporated herein by reference, entitled Photoresponsive Devices Containing Polyvinylsilicate Coatings, improved photoresponsive imaging members with a protective overcoating top layer of a crosslinked polyvinylsilicate resulting from the reaction of polysilicic acid with a polyvinyl alcohol with a number average molecular weight of from about 10,000 to about 100,000.
Several of the above-described overcoated organic photoresponsive devices are not effectively protected after extended usage, and in some instances the imaging properties thereof are adversely effected subsequent to a few imaging cycles. More specifically, with these devices the properties of the top overcoating material, or the properties of the other layers are usually adversely effected by ozone and other contaminants present in the environment by the developing compositions which contact the photoresponsive device for the purpose of rendering the image visible, and mechanical abrasion during cycling. Accordingly, images of low quality, or no images whatsoever are produced depending upon the extensiveness of the damage caused to the layers of the photoconductive device selected. Furthermore, in some instances, the toner materials employed do not sufficiently release from the photoresponsive surface, leaving unwanted toner particles thereon causing them to be subsequently embedded into, or transferred from the imaging surface in later imaging steps thereby resulting in undesirable images of low quality and/or high background. Also, in some instances, the dried toner particles adhere to the imaging member and print out as background areas. This can be particularly troublesome when known silicone resins or elastomeric polymers are employed as overcoating materials for their melted toner release characteristics since any low molecular weight components contained in these polymers can migrate to the surface of the silicone polymer layer, and act as an adhesive for dry toner particles brought in contact therewith during image development. There thus results undesirable high background areas in the final image since toner particles together with the developed images are effectively transferred to the receiving sheet.
Furthermore, illustrated in U.S. Pat. No. 4,562,132, the disclosure of which is totally incorporated herein by reference, entitled Photoresponsive Imaging Members Containing Electron Transport Overcoatings, are imaging members comprised of a supporting substrate, a hole transport layer comprised of an arylamine hole transporting compound dispersed in an inactive resinous binder, a photogenerating layer comprised of a photogenerating pigment optionally dispersed in a resinous binder, and as a protective topcoating an electron transporting compound of the following formula dispersed in a resinous binder ##STR2## where X is cyano or alkoxycarbonyl groups, A and B are electron withdrawing groups, m is a number of from 0 to 2, n is the number 0 or 1, and W is an electron withdrawing group selected from the group consisting of acyl (COR), alkoxycarbonyl (COOR), alkylaminocarbonyl (CONHR), and derivatives thereof.
Moreover, illustrated in copending application U.S. Ser. No. 882,117, entitled Phoresponsive Imaging Members With Electron Transport Overcoatings, the disclosure of which is totally incorporated herein by reference, are imaging members similar to those of the present application with the exception that there is selected for the overcoating of imaging members of the present invention certain novel copolyurethane overcoatings. More specifically, there are described in this copending application inorganic photoresponsive imaging members having incorporated herein as protective overcoatings electron transporting polycondensation polymers derived from the polycondensation of 2,2-bis(hydroxymethyl)butyl 9-dicyanomethylene-fluorene-4-carboxylate, and diisocyanate. Also disclosed in the copending application are layered photoresponsive imaging members comprised of a supporting substrate, a photoconductive layer, an arylamine hole transport layer, and a protective electron transporting overcoating layer comprised of the aforementioned polyurethane polymers. In addition, the electron transport polyurethane polymers of the copending application are useful as the top overcoating for positive-charging layered photoresponsive devices comprised of a supporting substrate, a hole transport layer, and a photoconductive layer, and wherein the polymers are of the following formula. ##STR3## wherein A is a trivalent linkage; B is a functional group such as an ester (--OCO--), a carbonate (--OCOO--) or a carbamate (--OCONH--); R is a bivalent group, and n represents a certain number of repeating units.
The copolyurethane overcoatings of the present invention are somewhat similar to the aforementioned polyurethane coatings, and further the aforementioned copolyurethanes have enhanced flexibility characteristics as compared to those polyurethanes illustrated in the U.S. Pat. No. 4,562,132. More specifically, the copolyurethanes of the present invention contain therein certain highly flexible segments enhancing its flexibility characteristics which is of particular importance when these polymers are selected as protective overcoatings for belt photoconductors, and moreover the copolyurethanes of the present invention are useful as a protectant for extended time periods. Furthermore, the presence of the soft flexible segments in the copolyurethanes of the present invention greatly improve their solubilities in common coating solvents such as aromatic hydrocarbons, tetrahydrofuran, chlorinated hydrocarbons, and the like, thereby enabling the coating process to be accomplished in a variety of solvents by different coating techniques, such as dip coating, spray coating, and the like. More importantly, the incorporation of the flexible segments into the polyurethane structure renders the synthesis of higher molecular-weight polyurethanes feasible, thus affording tough, highly durable polyurethanes for protective overcoating application.
Other prior art includes U.S. Pat. Nos. 4,474,865, which describes improved photoresponsive imaging members with electron transporting components containing specific dicyano fluoro ester moieties; 3,928,034, which illustrates the incorporation of electron transporting moieties chemically attached to polymers, reference columns 7 and 8; and 4,007,043; 4,063,947; 4,075,012; and 3,896,184. Also of interest are U.S. Pat. Nos. 3,108,092; 3,451,969; 4,063,947; and 4,203, Patent Publication No. 7606525. Of particular interest are U.S. Pat. No. 4,063,947 and Holland No. 7606525, which disclose imaging members with electron transport compounds, reference column 3, line 57, to column 4, line 30, of the U.S. Pat. No. 4,063,947.
While the above-described imaging members disclosed, particularly those of the pending application, are suitable for their intended purposes, there continues to be a need for improved protective overcoatings for incorporation into inorganic and organic imaging members. More specifically, there continues to be a need for protective overcoatings for inorganic imaging members, inclusive of selenium, and selenium alloys, which simultaneously function as charge transporting components enabling the resulting photoresponsive imaging members to be useful in xerographic imaging processes. Additionally, there continues to be a need for overcoatings which possess excellent toner release properties, and are impermeable to chemical materials produced by corona charging devices, and wherein the overcoatings selected are soluble in a variety of solvents thereby permitting improved coatability, and allowing economical spray and dip coating processes to be selected. There also continues to be a need for insulating protective overcoatings which are not conductive to charges applied by a corona charging device. Furthermore, there remains a need for protective overcoatings which are mechanically strong and durable while simultaneously being insensitive to the effect of humidity. Also, there is a need for heat resistant overcoatings for inorganic photoresponsive imaging members which are capable of protecting these members from direct exposure to heat without adversely effecting their imaging performance. There also remains a need for protective overcoatings which prevent the escape of toxic materials, especially inorganic materials such as arsenic and tellurium from photoreceptor imaging members. Moreover, there is a need for protective overcoatings that will prevent photoconductors such as selenium from cyrstallization upon exposure to chemical vapors. Further, there continues to be a need for new protective overcoatings for inorganic photoconductive members inclusive of members comprised of selenium and selenium alloys. Also, there is a need for reliable single component protective overcoatings for layered imaging members, which coatings have several desirable characteristics including toughness and high durability.